Previously I have examined the manner in which Kant uses his anti-skeptical transcendental argument to prove the existence of external objects. In this regard, he is less skeptical than his empiricists counterparts, such as Hume. However, as Kant represents a type of middle ground between the rationalists and the empiricists, there is also a way that he is more skeptical in his rationalism.
Kant’s idealist and empiricist epistemology required him to further distinguish between two types of knowledge one may have of external objects. Kant called objects that were posited by the mind without any sense perceptions noumena. Conversely, objects of the senses were called phenomena.
This all sounds very similar to Plato’s allegory of the cave. In Kantian terms, the Platonic noumenal realm would be the world of ideas and the phenomenal world the sensible reality. According to Plato per his allegory, the noumenal realm is more real and certain than the shadowy phenomenal realm.
Because Kant eschews forms and final causes, his version of a noumenon was different than that of Platonic or Aristotelian philosophies. Kant equates a noumenon as a “thing-in-itself”, whereas the phenomenon is merely an event or property manifested by an object and perceived by the senses. In short, Kant asserts that a phenomenon is a perceptive representation of an object existing in the mind of a perceiver, rather than the object in itself.
Here is the origin of Kant’s skepticism of the noumenal world. While Kant will not go as far to say that such a world does not exist, he contends that human knowledge of it is impossible. For Kant, the a priori knowledge or conditions that the mind uses to understand the external world exist only to “order” phenomena (which are merely mental appearances). While Kant will go further than Berkeley and admit that for there to be an appearance there must be something for there to be an appearance of, he maintains complete skepticism about knowledge of these substances in of themselves.
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